It has been suggested that the theoretical possibility of multiple births resulting from polar body fertilization, in addition to the normal mechanism of multi-ovular fertilization. This possibility has been investigated, and rejected, for normally conceived multiples, but not for multiples resulting from maternal use of fertility medication. This project proposes to investigate genetic relatedness in fertility drug, multiple births. Small blood samples will be drawn from fertility twins, their parents and sibs, and discordance noted in multiple blood groups. The observed discordance will be compared to the expected discordance, based on normal fraternal twinning.